A method for large-area application of mechanically sensitive layers onto a substrate.
The so-called liquid crystal displays (LCDs) dominate the market of the flat screen fields today. However, besides cost-effective manufacturing, low electrical uptake, low weight and low space requirements, the LCD technique also has severe disadvantages. LCDs do not emit themselves and therefore can be read easily or recognized only when the environmental lighting conditions are especially favorable. In most cases, this requires back-lighting, but this again increases the thickness of the flat screen several times. In addition, the predominant part of the electrical power uptake is then used for illumination and a higher voltage is needed for the operation of the lamps or fluorescent tubes. This is produced mostly with the aid of voltage-up converters from storage batteries. Another disadvantage is the highly limited angle of viewing of simple LCDs and the long switching times of individual pixels, which typically are in the range of a few milliseconds and are highly temperature-dependent. The delayed appearance of the image is extremely disturbing, especially when used in vehicles or in video applications.
Since 1987, displays based on organic light emitting diodes (OLEDs) have become known. These consist in principle of electroluminescent organic layers, which are arranged between two electrodes. When an electric potential is applied to the electrodes, emission of light then occurs due to the recombination between electrons and “holes”, which are injected into the organic layer.
OLEDs do not exhibit the disadvantages mentioned above. Due to self-emission, the need for back-lighting is eliminated, which reduces the space requirement and the electrical power uptake considerably. The switching times lie in the region of one microsecond and are only slightly temperature-dependent, which makes use for video applications possible. The reading angle is almost 180°. Polarization films, which are necessary for LCDs, are mostly eliminated, so that greater brightness of the display elements can be achieved. Another advantage is that flexible and nonplanar substrates can be utilized.
In the manufacture of OLEDs, low-molecular organic materials, for example, hydroxyquinoline aluminum(III) salts can be used, which are applied mostly by thermal evaporation onto a corresponding substrate. Displays based on this technology are already commercially available and, at this time, are used mainly in automobile electronics. However, since the manufacture of these components requires numerous process steps under high vacuum, this technology involves disadvantages due to high investment and maintenance costs, as well as relatively low throughput.
Therefore, an OLED technology was developed that uses polymers as organic materials, which can be applied from a solution onto the substrate using wet chemical methods. The vacuum steps necessary for producing the organic layers are eliminated with this technology. At the present time, the electroluminescent polymers are applied mostly with the aid of a rotary centrifugal method. This method has a number of disadvantages:
The majority of the polymer solution (about 99%) is lost irrevocably, the centrifuging process takes a relatively long time (about 30 to 60 seconds) and, moreover, it is almost impossible to apply homogeneous polymer layers onto large substrates.
In the OLEDs, frequently multilayer functional polymer layers are used that consist of, for example, hole transport polymers and emitter polymers. It is known from publication EP 0 892 028 A2 that functional layers can be applied into the window of a window layer with the aid of a contactless ink-jet printing method, which defines the pixels. However, with the aid of this contactless printing method, multilayer functional layers can also be produced. However, using the ink-jet printing method, smooth surfaces are very difficult to coat homogeneously. Moreover, ink-jet printing methods are very time-consuming and thus costly.
A number of standard printing methods are known from publication WO 99/07189, for example, a roll printing method, offset printing method, as well as screen printing method, for the application of electroluminescent polymers. These standard printing methods have the great advantage that, with them, functional layers can be applied onto large areas very rapidly and cost-effectively. However, problems arise with these standard printing methods when two or more functional polymer layers applied on top of one another or next to one another. In this case, the part of the printing device which is responsible for the transfer of the functional layers, for example, the screen, the template, the dabber or the roll, penetrates into the already applied mechanically sensitive polymer layer and damages it. Among others, the power of the OLED display produced in this way suffers, too, so that one must expect a shortening of the life and nonhomogeneous illumination of the display.